Many older “legacy” tightly-coupled navigation systems are designed to use data from older Global Positioning System (GPS) cards having a limited number of output data channels (satellite measurements). These legacy navigation systems cannot use all of the rich set of data available from current high-capability all-in-view GPS or GNSS (Global Navigation Satellite Systems). Legacy satellite navigation systems are incorporated into aircraft avionics. Avionics systems are subject to strict regulation and testing, and are therefore inconvenient to replace.
The typical approach to supporting these systems to pick a subset of satellite vehicles that gives the best dilution of precision. This effectively throws away satellite data that could be used to improve the performance and robustness of the system
Consequently, a technique is needed which can utilize all the data from a high-capability, multi-frequency, all-in-view GPS or GNSS navigation receiver but still be compatible with the limited capabilities of legacy navigation systems.